Zhuojin Pipe Fitting Co., LtdFailure Analysis of Inner Wall Cracks in 37Mn5 Hot-Rolled Steel Pipe
Literature Overview
The study by Cao Jingjing, Zhao Wenwu, Li Yang, and Li Hezong, published in Metal Heat Treatment (2016, Vol. 41, No. 1, pp. 228–232), presents a detailed failure analysis of inner wall cracks found in 37Mn5 hot-rolled steel pipes after quenching and tempering treatment. Funded by the Hebei Natural Science Foundation (Grant No. E2013402064), the research originates from the School of Mechanical and Electrical Engineering at Hebei University of Engineering.
37Mn5 steel is a dedicated grade for manufacturing J55-grade oil country tubular goods (OCTG), specifically casing and tubing used in petroleum drilling and production operations. Inner wall cracks in such pipes represent a critical quality defect that can lead to catastrophic failure under downhole conditions, making their root cause analysis essential for quality control and process improvement.
Failure Analysis Methodology
The investigation employed a comprehensive multi-scale analysis approach combining macroscopic and microscopic examination techniques:
Analytical Techniques Employed
| Technique | Purpose |
|---|---|
| Dye Penetrant Inspection (PT) | Crack detection and extent mapping |
| Optical Microscopy (OM) | Non-metallic inclusion identification and microstructure examination |
| Scanning Electron Microscopy (SEM) | Fracture surface morphology analysis |
| Energy Dispersive Spectroscopy (EDS) | Elemental composition of oxide inclusions |
Key Findings
The analysis revealed the following critical observations:
- Non-metallic inclusions were identified as sulfides with a fineness grade of 1.5 (indicating medium-sized inclusions).
- The inner wall cracks were relatively short in length but deep in penetration, oriented essentially perpendicular to the inner wall surface.
- Crack tips were sharp and fine, with oxide present within the crack interior.
- EDS analysis confirmed the oxide as iron oxide (Fe₂O₃/Fe₃O₄), indicating oxidation and decarbonization phenomena.
- The presence of oxide within the crack, combined with the fracture morphology, definitively classified the cracks as non-quench cracks.
Root Cause Determination
Based on the metallurgical evidence, the authors concluded that the cracks originated during the piercing and rolling process (穿轧) and subsequently propagated under the influence of quenching stresses. The mechanism can be summarized as follows:
- During piercing and rolling, internal defects such as shrinkage cavities or oxide inclusions were introduced into the hot-rolled pipe body.
- These defects acted as stress concentrators and crack initiation sites.
- During the subsequent quenching and tempering treatment, thermal stresses and transformation stresses (due to martensitic transformation) provided the driving force for crack propagation.
- The cracks propagated perpendicular to the inner wall, reaching significant depths before being arrested by the tempering process.
Defect Classification and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Inner wall longitudinal cracks | Piercing/rolling defects + quenching stress propagation | Improve piercing conditions; reduce quenching severity; pre-heat before quenching |
| Sulfide inclusions (Grade 1.5) | Inadequate desulfurization during steelmaking | Enhance secondary refining; optimize deoxidation practice |
| Oxide within crack | High-temperature oxidation during hot working | Improve rolling temperature control; minimize exposure time at elevated temperatures |
Engineering Practice Implications
For steel pipe manufacturing quality control, this failure analysis underscores the critical importance of controlling internal defects during the hot rolling process. The FMEA (Failure Mode and Effects Analysis) approach applied here demonstrates that a systematic investigation combining multiple analytical techniques is essential for accurate root cause determination.
Key process control points identified from this analysis include:
- Steelmaking: Ensure adequate desulfurization to minimize sulfide inclusion formation.
- Hot rolling: Optimize piercing conditions to prevent internal crack initiation.
- Heat treatment: Control quenching severity to limit thermal and transformation stresses.
- Inspection: Implement internal defect detection methods (UT, ECT) prior to heat treatment to identify and reject affected material.
Summary and Reflections
This failure analysis provides a clear demonstration of how metallurgical examination techniques can be systematically applied to trace defects back to their manufacturing origin. The finding that cracks formed during hot rolling and propagated during quenching highlights the interdependence of manufacturing processes and the necessity of integrated quality control throughout the entire production chain. For engineers responsible for steel pipe quality assurance, this case study reinforces the principle that defects detected at a late stage often originate from earlier process steps, and effective prevention requires upstream process optimization rather than downstream inspection alone.